This section is intended to introduce the reader to various aspects of art, which may be associated with exemplary embodiments of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with information to facilitate a better understanding of particular techniques of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not necessarily as admissions of prior art.
Cargo is generally transferred from one port location to another port location by vessels, such as carriers. These carriers have propulsion and navigation systems for movement across large bodies of water, which may be referred to as open seas. In addition, the carriers may include accommodations for marine operations, storage tanks for liquid cargo and bays for solid cargo. With some carriers, special equipment and systems may be installed to assist with the transport of specific cargo. As such, carriers include equipment and systems to economically transfer cargo between market locations.
For instance, after natural gas is produced, it is processed and may be liquefied at export terminals or other facilities to convert it into LNG. LNG is the basis of a delivery technology that allows remote natural gas resources to be economically delivered to the market. The LNG is shipped to market in specially-designed LNG carriers (LNGCs) that are configured to store and transport the LNG across the large bodies of water. Then, the LNG is converted back into natural gas at an import terminal near the market location. Typically, the import terminals are located onshore or offshore near a port location. Regardless, the import terminal is connected through a pipeline to onshore equipment for further processing and/or distribution of the natural gas.
Offshore import or export terminals may be beneficial because they do not utilize onshore property, which may reduce some security concerns. However, significant technical challenges need to be addressed to successfully implement offshore terminals. An example of an offshore LNG import terminal is a floating storage and regasification unit (FSRU). An FSRU is a dedicated, moored offshore structure that transfers LNG from LNGCs, stores the LNG in storage tanks, regasifies the LNG using heat exchangers, and delivers the natural gas to a pipeline. An FSRU generally includes cryogenic cargo transfer equipment and LNG vaporization facilities, which may be located on the deck of the FSRU.
Further, offshore environmental conditions are a factor that limit the time periods that the LNGCs are able to offload LNG into an FSRU. For instance, harsh environmental conditions may provide periods of time where connecting the LNGCs and FSRU cannot be done safely and reliably. Further, if the offshore environmental conditions are too severe to allow the LNGCs and FSRU to connect, then the FSRU can only deliver natural gas to the pipeline from its stored reserves. Because of this, stored reserves on the FSRU may become depleted, leading to an interruption of natural gas delivery to the pipeline. Intermittent service or interruptions to the flow of natural gas into or from a pipeline may result in penalties and cost increases for companies operating the import or export terminals.
To address the environmental conditions, various offloading approaches are utilized to transfer LNG between LNGCs and FSRUs. For instance, one offloading approach is side-by-side offloading, which is currently employed at land-based import and export terminals. Side-by-side offloading may be performed with the LNGC and FSRU arranged in a side-by-side configuration with the LNG transfer occurring using marinized mechanical loading arms located near amidships of each LNGC and FSRU. Conventional land-based cargo transfer using mechanical loading arms is typically performed in protected waters.
A second offloading approach is tandem offloading. Tandem offloading of LNG parallels existing technology used to transfer oil between floating production storage and offloading (FPSO) vessels and shuttle tankers. Typically, the two vessels are arranged bow-to-stern with the cargo transfer achieved using flexible hoses. For LNG transfer, flexible cryogenic hoses or large loading arms, which are called booms, may be utilized with the LNGCs carrier bow located behind the stern of the FSRU. With these flexible cryogenic hoses or large loading arms, the tandem offloading approach may remain operable in more severe seastates than the side-by-side offloading approach.
A third offloading approach employs a subsea cryogenic fluid transfer system, which is described in International Patent Application No. W02006/044053. In this offloading approach, the LNGC and FSRU are connected over a distance of about 2 kilometers (km) by cryogenic turrets, risers and pipelines. The LNGC is connected to a submerged, disconnectable cryogenic buoy and transfers the LNG through this buoy and one or more flexible cryogenic risers to the seafloor, over to the FSRU location through one or more cryogenic pipelines, up one or more flexible cryogenic risers and into the FSRU through a cryogenic internal turret mooring system. Because the LNGC and FSRU are separated and may move independently, this offloading system may operate in extreme seastates, such as 4 to 5 meter significant wave heights.
While each of these offloading approaches may be utilized to maintain uniform delivery of natural gas to the pipeline, the use of FSRUs with any of these offloading approaches suffers from technical and commercial limitations. For instance, because FSRUs are permanently moored with no access to drydock maintenance, numerous upgrades are made to ensure that the facility remains operable over the project lifetime, which results in significant capital expenditure. Examples of these upgrades include additional hull steel for lengthening of fatigue life, improved hull coatings for corrosion resistance, and additional provisions for onsite inspections. This large initial capital expenditure results in a significant reduction in the overall LNG delivery chain economics. Also, additional equipment and operations, such as dedicated positioning tugs or navigation systems on the LNGCs, are involved to facilitate berthing operations for the LNGCs with the FSRU. While improved relative to onshore terminals, FSRUs still pose a security threat and have to be managed to address the open access provided in an offshore setting.
An alternate to the FSRU-based LNG import terminal is to include the regasification equipment on the LNGC. See U.S. Pat. No. 6,089,022. These vessels are LNGCs with extensive modifications to allow shipboard regasification of the LNG and offloading of the natural gas into the pipeline. These carriers, which may be referred to as Shipboard Regasification Terminals (SRTs), are equipped with regasification equipment and traditional LNGC offloading equipment (i.e. a manifold to accept loading arms) to interact with conventional LNG terminals. Disadvantageously, the capital expense of these SRTs may be greater than traditional LNGCs because each SRT vessel is modified with heat exchangers for regasification operations, a natural gas offloading system, and reinforced LNG cargo tanks to withstand sloshing loads. Because of these additional capital expenses, using only SRTs to deliver LNG tends to be uneconomic for long distances and/or large volumes. In addition, the LNG storage on the SRTs is somewhat limited because these vessels are designed for efficient transit over long distances.
As such, a method or mechanism for enhancing delivery of cargo, such as LNG, in an efficient manner is needed. This efficient method or mechanism may ideally alleviate the issues associated with operating offshore LNG import terminals.
Other related material may be found in at least U.S. Pat. No. 3,590,407; U.S. Pat. No. 5,501,625; U.S. Pat. No. 5,549,164; U.S. Pat. No. 6,003,603; U.S. Pat. No. 6,089,022; U.S. Pat. No. 6,637,479; U.S. Pat. No. 6,923,225; U.S. Pat. No. 7,080,673; U.S. Patent Application Publication No. 2002/0174662; U.S. Patent Application Publication No. 2004/0187385; U.S. Patent Application Publication No. 2006/0010911; European Patent Application No. 1,383,676; International Patent Application No. WO 01/03793; International Patent Application No. W02006/044053; Loez, Bernard “New Technical and Economic Aspects of LNG Terminals,” Petrole Information, pp. 85-86, August 1987; Hans Y. S. Han et al., “Design Development of FSRU from LNG Carrier and FPSO Construction Experiences,” Offshore Technology Conference May 6-9, 2002, OTC-14098; “The Application of the FSRU for LNG Imports,” Annual GAP Europe Chapter Meeting Sep. 25-26, 2003; and O. B. Larsen et al., “The LNG (Liquefied Natural Gas) Shuttle and Regas Vessel System,” Offshore Technology Conference May 3-6, 2004, OTC-16580.